Recommendations on apache tuning for chat application

Question

Hi,

I have a chat application (xmpp / muc) that is going to be served by apache (we might change to nginx later but right now it's not easily done). If a user is in 2 rooms, he'll have between 2 and 4 active connections to the server (long-polling connections), so if we have 200 users per room and we have 5 rooms, what should the ServerLimit, MaxClients be set to? For example, these are the default values:

ServerLimit 256

MaxClients 256

MaxRequestsPerChild 4000

Thanks,

Answer 1

I'm guessing this a pre-fork Apache....

If you've got 1000 concurrent chat connections, then you need at least 1000 webservers (presumably you'll be serving up static content too, and you say that each presence results in 2 connections - so you can double the 1000) - a ServerLimit/MaxClients of 256 is not going to cut it, you probably need around 2200 to support this (but without hard metrics its hard to give an exact figure).

This is a ridiculously large amount. To support this I'd be looking for 3 boxes, each with about 2Gb free memory before the webserver is started.

MaxRequestsPerChild is not really relevant other than the fact you want some turnover of processes, particularly if you are using long polling.

This is one of the reasons why COMET is not a good idea. Using AJAX polling would be much more efficient. Assuming this is not something you can change, you might want to have a look at using a threaded Apache webserver which is marginally more memory efficient,

C.

Answer 2

Each user should only have a single connection to the XMPP server if your using something like BOSH to make the Web->XMPP connection. Each MUC room activity will be sent to the XMPP server with the user's JID as the target and that will cause the server to send that to the appropriate BOSH connection. Same for presence.

The information you need to load balance the Apache setup is the same as load balancing any long-polling connection - XMPP doesn't add anything exotic to the mix. An XMPP BOSH connection is a long-polling connection to your web server which is then sent to a persistent connection to the XMPP server - causing 2 sockets to be opened per user connect.